The present invention relates to a photocoupler for transmitting, at high speed, a signal insulated, or isolated, by the photocoupler and particularly to a photocoupler circuit for driving a power semiconductor element at high speed.
For power conversion with ON-OFF operation of a semiconductor element, the ON-OFF signal to be applied to such semiconductor element is insulated and transmitted by a photocoupler. The power conversion, due to pulse width modulation control, is required to form a photocoupler circuit to satisfy the requirement that such ON-OFF signal must be transmitted at a high speed.
FIG. 12 is a schematic diagram showing a prior art circuit for transmitting a signal using a photocoupler.
In the circuit of FIG. 12, a pair of general purpose photocouplers 2 are used to insulate and transmit the ON-OFF signal to a power transistor 20. General purpose photocouplers use phototransistors to receive the light; these phototransistors have a long transition time from the ON state to the OFF state A long transition time from ON to OFF of such phototransistor is considerably long because a reasonably long time is required to eliminate excessive carrier accumulated in this phototransistor.
FIG. 13 is a schematic diagram showing a second prior art circuit for transmitting a signal using a photocoupler.
In the second prior art circuit of FIG. 13, a high speed photocoupler 2A is used for insulating and transmitting a signal; the pair of general purpose photocouplers used in the first prior art of FIG. 12 is now reduced to a single high speed unit.
As is apparent from the first and second prior art circuits described above, these prior art circuits have disadvantages. First, the circuit using general purpose photocouplers is complicated, and the number of parts increases since a pair of photocouplers (for start and stop) must be used to realize high speed signal transmission. Second, a comparatively large drive current is necessary to make the photocouplers conductive.
Moreover, if a high speed photocoupler is used, other disadvantages occur. A high speed photocoupler is expensive; it is large in comparison with a general purpose photocoupler; and it requires a comparatively large drive current to become conductive.
It is therefore an object of the present invention to realize high speed signal transmission by reducing the number of general purpose and low speed photocouplers while using them and also reduce the required drive current.